Iron coordination chemistry of N-(bis(2-pyridyl)methyl)pyridine-2-carboxamide

The amidate function participates in the coordination chemistry of iron containing biomolecules such as the anti-tumor drug bleomycin and the enzyme nitrile hydratase. Our interest in amidate coordination prompted an investigation of the iron complexes of the potentially tetradentate ligand N-(bis(2-pyridyl)methyl)pyridine-2-carboxamide (H-L). A number of complexes have been isolated and structurally characterized, including [Fe^II(L)₂] (1), [Fe^III(L-OCH₃)Br₂(CH₃OH)] (3), [Fe₂^III(μ-OH)₂(L-OCH₃) ₂Br₂] (4), and [Fe₄^III(μ-OCH₃)₂(L-O) ₂Br₆] (5). In these complexes L acts as a meridional tridentate ligand, as previously observed for the corresponding [Cu(L)Cl(CH₃OH)] complex (Inorg. Chem. 39 (2000) 5326). In the cases of 3 and 4, the hydrogen of the tertiary carbon has been replaced by a methoxy group in the course of complex synthesis. In the case of 5, the tertiary hydrogen is replaced by hydroxide, and this oxygen and the dangling pyridine act as a bidentate ligand to a second iron ion. When the reaction of FeBr₃ and H-L was carried out in acetonitrile in the presence of base but in the absence of air, the ligand was cleaved into two pieces, affording [Fe^IIBr₂(pyridine-2-carboxamide)(di-2-pyridylketone)] (6). It is proposed that the coordination of the amide nitrogen of H-L to an iron(III) center as an amidate activates the α-C-H bond and results in the oxidation of the α-C-N_amide bond to an imine.